This invention relates to a substantially tubular insert which is capable of collapsing in an axial direction in response to a compressive force exerted by a fastening element, and to an assembly incorporating said insert as an integral component thereof. Upon compression of the insert, a considerable portion of the net compressive load exerted by the fastener is absorbed by the insert without excessive loading on the article.
As is well-known in the fastener art, the problem of maintaining adequate compression or torque when assembling metal or non-metal parts to other assemblies having dissimilar coefficients of thermal expansion has persisted in industry for many years and has previously denied the acceptance of materials in fields of application in which they would otherwise find great utility. A part which is subjected to a compressive load due to the clamping action of a fastener will tend to deform over a period of time, as a result of a condition known as "cold flow", i.e. permanent deformation resulting from prolonged application of a stress below the elastic limit of a material. This condition is further aggravated when an assembly is thermally cycled and the part tends to undergo a greater expansion than the metal fastener will permit due to the dissimilarities in thermal expansion coefficients. This is especially true, for instance, in the automotive industry where assemblies are frequently exposed to severe temperature cycles during which the materials are alternately subjected to thermal expansion and contraction.
In the past, various types of inserts have been employed in plastic articles either by press-fitting the insert into a finished molding or by placing the insert in the cavity so that it becomes an integral part of the molding. Typical inserts of this type include a wide variety of internally threaded bushings used for attaching a molded plastic article to another object. By way of example, U.S. Pat. No. 2,591,575 to McCollum, describes an internally-threaded, expandable fastener insert for use in molded plastic articles adapted to be anchored to the plastic article by means of which other parts may be fastened thereto.
This and other similar prior art inserts are distinguished from the insert of the present invention which is not internally threaded to engage a fastener and is not adapted to anchor the part by expanding outwardly in a radial direction in response to compressive loading by a fastener. Rather, the insert of this invention is designed to collapse uniformly in an axial direction without exerting lateral forces on the part due to lateral motion of the external surface. In this manner, the insert described herein will prevent substantial deformation of the part while maintaining the specified compressive loading on the assembly.
Accordingly, it is a principal object of the instant invention to provide a means for mounting parts onto a workpiece by employing a collapsible tubular insert in the part which is adapted to absorb a substantial amount of the total compressive load exerted upon tightening a fastener.